Lentivirus infections are generally characterized by their chronic and degenerative clinical course in which the virus is able to persist despite a significant host immune response. Two animal models, equine infectious anemia virus (EIAV) and caprine arthritis-encephalitis virus (CAEV) are being studied in order to determine the molecular basis of viral pathogenesis. Analysis of cDNA libraries of cells chronically infected with EIAV revealed an array of transcripts that could encode viral structural or regulatory proteins. Studies focusing on cDNAs capable of trans-activating the EIAV LTR demonstrated that EIAV expresses at least three distinct tat mRNAs with complex structures. Sequence analysis and in vitro mutagenesis studies confirmed and extended earlier findings that the EIAV tat gene initiates translation at a non-Aug codon. The tat mRNAs were predicted to be polycistronic, containing open reading frames (orfs) downstream of tat that could encode a putative rev protein and/or two different versions of amino-terminally truncated forms of the transmembrane protein. cDNA constructs or subgenomic clones containing only the tat orf were reproducibly more active than polycistronic forms in trans-activation assays. The EIAV tat protein was detected for the first time as an 8 Kd species in cells transfected with tat cDNAs or in chronically infected cells. An infectious molecular EIAV clone was isolated but virus derived from it could not induce clinical symptoms, although it could persistently infect horses. Studies are in progress to isolate portions of the genome of the pathogenic EIAV "field strain" directly from blood cells of diseased horses by PCR techniques. Chimeras between these segments and the infectious clone will be generated and virus derived from them will be tested for pathogenicity. The pattern of expression of CAEV in acutely infected cells was found to be complex and temporally regulated. Nucleotide sequence analysis suggested that they represent rev-like transcripts which could potentially encode as well a truncated form of the env transmembrane protein and a novel protein, designated X, composed of 73 amino acids.